This invention relates to copolymers of polybutylene terephthalate which have an improved 264 psi heat deflection temperature. More specifically, this invention relates to polybutylene terephthalate having incorporated therein side chains of polystyrene and to a method for increasing the heat deflection temperature of polybutylene terephthalate.
Polybutylene terephthalate (PBT) is a crystalline polyester having good strength and toughness, good chemical and abrasion resistance and good electrical properties. The resin is readily injection moldable and finds wide application in the electrical and automotive fields. Unfilled PBT has a very high heat deflection temperature at low levels of stress. With increased stress levels, the heat deflection temperature of unfilled PBT drops sharply and parts molded from unfilled PBT cannot be employed at temperatures much above 100.degree. F. where significant stress levels will be encountered.
Reinforcement of PBT with glass fibers enhances the high temperature properties of PBT, increasing the 264 psi heat deflection temperature from 130.degree. F. for the unfilled resin to 415.degree. F. for glass fiber-filled PBT. Addition of glass fiber also increases such mechanical properties as tensile strength and modulus and lowers overall elongation. PBT has a sharp crystalline melting point of 435.degree. F. and is readily processed at melt temperatures above 450.degree. F. This good flowability of PBT permits rapid injection molding at moderate injection pressures even with large and intricate molds. The addition of fillers such as glass fiber generally decreases flowability while increasing the heat deflection temperature of PBT. Additionally, the use of glass fiber fillers in some applications such as in film and filament extrusion is not generally desirable.
A method for improving the high stress heat deflection temperature of PBT without detrimentally affecting the otherwise excellent processing characteristics of PBT would increase the range of potential end uses for unfilled PBT.